Air-conditioning apparatuses include one in which, like a multi-air-conditioning apparatus for buildings, a heat source (outdoor unit) is installed outside a building and an indoor unit is installed inside the building. A refrigerant that circulates in a refrigerant circuit of the air-conditioning apparatus transfers heat to (or receives heat from) air supplied to a heat exchanger of the indoor unit so as to heat or cool the air. Then, the heated or cooled air is sent to an air-conditioned space for heating or cooling the space.
Such an air-conditioning apparatus often includes a plurality of indoor units, because a building typically has a plurality of indoor spaces. In the case of a large building, a refrigerant pipe that connects the outdoor unit and each indoor unit may reach as long as 100 m. The longer the pipe that connects the outdoor unit and the indoor unit, the larger the amount of refrigerant charged into the refrigerant circuit.
An indoor unit of such a multi-air-conditioning apparatus for buildings is typically installed and used in an indoor space (e.g., office space, room, or shop) where there are people. If for some reason a refrigerant leaks from the indoor unit installed in the indoor space, since the refrigerant may be flammable or toxic depending on its type, the leakage may cause safety or health problems. Even if the refrigerant is harmless to the human body, the leakage of the refrigerant may lower the concentration of oxygen in the indoor space and negatively affect the human body.
As a solution to this, an air-conditioning apparatus may use a secondary loop method in which, for air-conditioning of a space where there are people, a primary-side loop is performed with a refrigerant and a secondary-side loop is performed with harmless water or brine.
For prevention of global warming, there has been a demand for development of air-conditioning apparatuses that use a refrigerant with a low global warming potential (hereinafter may also be referred to as GWP). Promising low GWP refrigerants include R32, HFO1234yf, and HFO1234ze. Adopting only R32 as a refrigerant does not involve significant design changes to the current apparatus and requires only a small development load, because R32 has substantially the same physical properties as R410A which is currently most often used. However, R32 has a GWP of 675, which is a little high. On the other hand, if HFO1234yf or HFO1234ze alone is adopted as a refrigerant, the pressure of the refrigerant is low because of its small density in a low-pressure state (gas state or two-phase gas-liquid state), and thus the loss of pressure increases. However, increasing the diameter (inside diameter) of a refrigerant pipe to reduce the loss of pressure leads to a higher cost.
By using a mixture of R32 and HFO1234yf or HFO1234ze as a refrigerant, it is possible to reduce the GWP while increasing the pressure of the refrigerant. Since R32, HFO1234yf, and HFO1234ze have different boiling points, the resulting refrigerant mixture is a non-azeotropic refrigerant mixture.
It is known that in an air-conditioning apparatus using a non-azeotropic refrigerant mixture, the composition of the refrigerant charged in the apparatus is different from the composition of the refrigerant actually circulating in the refrigeration cycle. This is because the boiling points of the mixed refrigerants are different as described above. The change in refrigerant composition during circulation causes the degree of superheat or subcooling to deviate from the original value, makes it difficult to optimally control the opening degree of an expansion device and various other devices, and leads to degraded performance of the air-conditioning apparatus. To reduce such performance degradation, various refrigerating and air-conditioning apparatuses with means for detecting a refrigerant composition have been proposed (see, e.g., Patent Literatures 1 and 2).
The technique described in Patent Literature 1 includes a bypass that is connected to bypass a compressor, and a double-pipe heat exchanger and a capillary tube are connected to the bypass. A refrigerant composition is calculated on the basis of detection results of various detecting means included in the bypass and a refrigerant composition tentatively set. To determine the refrigerant composition, the technique described in Patent Literature 1 performs repetitive calculations until the calculated refrigerant composition satisfies a condition specified in a control flow.
Like the technique described in Patent Literature 1, the technique described in Patent Literature 2 involves setting a tentative refrigerant composition and calculating a refrigerant composition by repetitive calculations. The technique described in Patent Literature 2 includes a calculation flow for eliminating repetitive calculations.